Organic electroluminescence device

ABSTRACT

An organic EL device includes a pair of electrodes and an organic compound layer between pair of electrodes. The organic compound layer includes an emitting layer including a first material and a second material. The second material is a fluorescent material. Singlet energy EgS(H) of the first material and singlet energy EgS(D) of the second material satisfy a relationship of the following formula (1). The first material satisfies a relationship of the following formula (2) in terms of a difference ΔST(H) between the singlet energy EgS(H) and an energy gap Eg 77K (H) at 77K. 
       EgS( H )&gt;EgS( D )  (1)
 
       Δ ST ( H )=EgS( H )−Eg 77K ( H )&lt;0.3 (eV)  (2)

This application is a Continuation of U.S. Non-Provisional applicationSer. No. 14/141,364, which was filed on Dec. 26, 2013. U.S.Non-Provisional application Ser. No. 14/141,364 claims to benefit orpriority to U.S. Provisional Application No. 61/467,703, which was filedon Mar. 25, 2011. This application is based upon and claims the benefitof priority to Japanese Patent Application No. 2011-068758, which wasfiled Mar. 25 2011. The entire contents of Japanese Patent ApplicationNo. 2011-068758 and U.S. Provisional Application No. 61/467,703 areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescence device.

2. Description of Related Art

When voltage is applied on an organic electroluminescence device(hereinafter, referred to as an organic EL device), holes and electronsare respectively injected into an emitting layer from an anode and acathode. The injected electrons and holes are recombined in an emittinglayer to form excitons. Here, according to the electron spin statisticstheory, singlet excitons and triplet excitons are generated at a ratioof 25%:75%. In the classification according to the emission principle,in a fluorescent EL device which uses emission caused by singletexcitons, the limited value of an internal quantum efficiency of theorganic EL device is believed to be 25%. On the other hand, in aphosphorescent EL device which uses emission caused by triplet excitons,it has been known that the internal quantum efficiency can be improvedup to 100% when intersystem crossing efficiently occurs from the singletexcitons.

A technology for extending a lifetime of a fluorescent organic EL devicehas recently been improved and applied to a full-color display of amobile phone, TV and the like. However, an efficiency of a fluorescentEL device is required to be improved.

Based on such a background, a highly efficient fluorescent organic ELdevice using delayed fluorescence has been proposed and developed. Forinstance, Document 1 (International Publication No. WO2010/134350)discloses an organic EL device using TTF (Triplet-Triplet Fusion)mechanism that is one of mechanisms for delayed fluorescence. The TTFmechanism utilizes a phenomenon in which singlet excitons are generatedby collision between two triplet excitons.

By using delayed fluorescence by the TTF mechanism, it is consideredthat an internal quantum efficiency can be theoretically raised up to40% even in fluorescent emission. However, as compared withphosphorescent emission, the fluorescent emission is still problematicon improving efficiency. Accordingly, in order to enhance the internalquantum efficiency, an organic EL device using another delayedfluorescence mechanism has been studied.

For instance, TADF (Thermally Activated Delayed Fluorescence) mechanismis used. The TADF mechanism utilizes a phenomenon in which inverseintersystem crossing from triplet excitons to singlet excitons isgenerated by using a material having a small energy gap (ΔST) betweenthe singlet level and the triplet level. An organic EL device using theTADF mechanism is disclosed in Document 2: “Expression ofHighly-Efficient Thermally-Activated Delayed-Fluorescence andApplication thereof to OLED” Organic EL Symposium, proceeding for thetenth meeting edited by Chihaya Adachi et al., pp. 11-12, Jun. 17-18,2010. In the organic EL device of Document 2, a material having a smallΔST is used as a dopant material to cause inverse intersystem crossingfrom the triplet level to the singlet level by heat energy. It isconsidered that the internal quantum efficiency can be theoreticallyraised up to 100% even in fluorescent emission by using delayedfluorescence by the TADF mechanism,

Although the organic EL device disclosed in Document 2 exhibits themaximum luminous efficiency at 0.01 mA/cm² of a low current densityarea, so-called roll-off is generated to decrease a luminous efficiencyin a practically high current density area from approximately 1 mA/cm²to 10 mA/cm².

Accordingly, it is considered that many practical problems in usingdelayed fluorescence by the TADF mechanism are left unsolved, amongwhich improvement in the luminous efficiency in the practically highcurrent density area has been particularly demanded.

SUMMARY OF THE INVENTION

An object of the invention is to provide an organic EL deviceefficiently emitting light even in a practically high current densityarea using the TADF mechanism in which a material having a small ΔST isemployed.

After conducting concentrated studies in order to solve the aboveproblem, the inventors found that the organic EL device efficientlyemits light even in a high current density area by containing a firstmaterial and a second material in an emitting layer in which the firstmaterial is a compound satisfying specific conditions and the secondmaterial is a fluorescent material, and arrived at the invention.

An organic EL device according to an aspect of the invention includes apair of electrodes and an organic compound layer between the pair ofelectrodes, the organic compound layer comprising an emitting layercomprising a first material and a second material, in which the secondmaterial is a fluorescent material, singlet energy EgS(H) of the firstmaterial and singlet energy EgS(D) of the second material satisfy arelationship of a formula (1) below, and the first material satisfies arelationship of a formula (2) below in terms of a difference ΔST(H)between the singlet energy EgS(H) and an energy gap Eg_(77K)(H) at 77K.

EgS>EgS(D)  (1)

ΔST(H)=EgS(H)−Eg_(77K)(H)<0.3 [eV]  (2)

In the organic EL device according to the above aspect of the invention,it is preferable that the organic EL device exhibits a delayedfluorescence ratio larger than 37.5%.

The delayed fluorescence ratio is equivalent to a ratio of a luminousintensity derived from delayed fluorescence relative to the totalluminous intensity. Specifically, the delayed fluorescence ratio isobtained according to a calculation method described below.

In the above aspect of the invention, it is preferable that the organicEL device exhibits a residual intensity ratio larger than 36.0% afterthe elapse of 1 μs after voltage removal in a transitional ELmeasurement.

In the organic EL device according to the above aspect of the invention,it is preferable that a half bandwidth of a photoluminescence spectrumof the first material is 50 nm or more.

Further, in the organic EL device according to the above aspect of theinvention, it is preferable that a half bandwidth of a photoluminescencespectrum of the first material is 75 nm or more.

In the organic EL device according to the above aspect of the invention,it is preferable that a difference ΔT between the energy gap Eg_(77K)(H)at 77K of the first material and an energy gap Eg_(77K)(D) at 77K of thesecond material satisfies a relationship of a formula (3) below.

ΔT=Eg_(77K)(H)−Eg_(77K)(D)≧0.6 [eV]  (3)

An organic EL device of the invention efficiently emits light even in apractically high current density area using the TADF mechanism in whicha material having a small ΔST is employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an exemplary arrangement of an organic ELdevice according to an exemplary embodiment of the invention.

FIG. 2 shows an example of physics models with aggregate formation.

FIG. 3 shows a relationship in energy level between a host material anda dopant material in an emitting layer.

FIG. 4 shows a relationship in energy level between the host materialand the dopant material in the emitting layer.

FIG. 5 shows a measurement system of transitional EL waves.

FIG. 6A shows a measurement method of a ratio of luminous intensitiesderived from delayed fluorescence and is a graph showing time-varyingluminous intensities of the EL device.

FIG. 6B shows a measurement method of a ratio of luminous intensitiesderived from delayed fluorescence and is a graph showing time-varyinginverse square root of luminous intensities.

FIG. 7 shows a relationship in energy level between the host materialand the dopant material in the emitting layer.

FIG. 8A schematically shows an incident angle of an incident light froma light source as an example of spectroscopic ellipsometry measurement.

FIG. 8B shows a cross section of an organic thin film on a siliconsubstrate (a measurement target) as an example of the spectroscopicellipsometry measurement

FIG. 9 is a graph showing a relationship between a current efficiencyand a current density.

FIG. 10 is a graph showing time-varying luminous intensities of the ELdevice.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Arrangement(s) of OrganicEL Device

Arrangement(s) of an organic EL device according to the invention willbe described below.

The organic EL device according to the exemplary embodiment includes apair of electrodes and an organic compound layer between the pair ofelectrodes. The organic compound layer includes at least one layerformed of an organic compound. The organic compound layer may include aninorganic compound.

In the organic EL device according to the exemplary embodiment, at leastone layer of the organic compound layer includes an emitting layer.Accordingly, the organic compound layer may be provided by a singleemitting layer. Alternatively, the organic compound layer may beprovided by layers applied in a known organic EL device such as a holeinjecting layer, a hole transporting layer, an electron injecting layer,an electron transporting layer, a hole blocking layer, an electronblocking layer.

The followings are representative arrangement examples of an organic ELdevice:

(a) anode/emitting layer/cathode;(b) anode/hole injecting•transporting layer/emitting layer/cathode;(c) anode/emitting layer/electron injecting•transporting layer/cathode;(d) anode/hole injecting•transporting layer/emitting layer/electroninjecting-transporting layer/cathode; and(e) anode/hole injecting•transporting layer/emitting layer/blockinglayer/electron injecting•transporting layer/cathode.

While the arrangement (d) is preferably used among the abovearrangements, the arrangement of the invention is not limited to theabove arrangements.

It should be noted that the aforementioned “emitting layer” is anorganic compound layer generally employing a doping system and includinga first material and a second material. In general, the first materialpromotes recombination of electrons and holes and transmits excitationenergy generated by recombination to the second material. The firstmaterial is often referred to as a host material. Accordingly, the firstmaterial is referred to as the host material in descriptionshereinafter. In general, the second material receives excitation energyfrom the host material (the first material) to exhibit a highluminescent performance. The second material is often referred to as adopant material. Accordingly, the second material is referred to as thedopant material in descriptions hereinafter. The dopant material ispreferably a compound having a high quantum efficiency. In the exemplaryembodiment, a fluorescent material is used as the dopant material.

The “hole injecting/transporting layer (or hole injecting•transportinglayer) means “at least one of a hole injecting layer and a holetransporting layer while the “electron injecting/transporting layer (orelectron injecting transporting layer) means “at least one of anelectron injecting layer and an electron transporting layer. Herein,when the hole injecting layer and the hole transporting layer areprovided, the hole injecting layer is preferably close to the anode.When the electron injecting layer and the electron transporting layerare provided, the electron injecting layer is preferably close to thecathode.

In the exemplary embodiment, the electron transporting layer means anorganic layer having the highest electron mobility among organiclayer(s) providing an electron transporting zone existing between theemitting layer and the cathode. When the electron transporting zone isprovided by a single layer, the single layer is the electrontransporting layer. Moreover, a blocking layer having an electronmobility that is not always high may be provided as shown in thearrangement (e) between the emitting layer and the electron transportinglayer in order to prevent diffusion of excitation energy generated inthe emitting layer. Thus, the organic layer adjacent to the emittinglayer is not always an electron transporting layer.

FIG. 1 schematically shows an exemplary arrangement of an organic ELdevice according to an exemplary embodiment of the invention.

An organic electroluminescence device 1 includes a light-transmissivesubstrate 2, an anode 3, a cathode 4 and an organic compound layer 10disposed between the anode 3 and the cathode 4.

The organic compound layer 10 includes an emitting layer 5 containing ahost material and a dopant material. The organic compound layer 10 alsoincludes a hole injecting layer 6 and a hole transporting layer 7between the emitting layer 5 and the anode 3 in sequence from the anode3. The organic compound layer 10 further includes an electrontransporting layer 8 and an electron injecting layer 9 between theemitting layer 5 and the cathode 4 in sequence from the emitting layer5.

Emitting Layer

In this exemplary embodiment, as described above, a compound satisfyingspecific conditions is used as the host material and the dopant materialof the emitting layer. The specific conditions will be described below.ΔST

The inventors found that the organic EL device emits light at a highefficiency in a high current density area by using a compound having asmall energy gap (ΔST) between singlet energy EgS and triplet energy EgTas the host material. The ΔST(H) refers to ΔST of the host material.

From quantum chemical viewpoint, decrease in the energy difference (ΔST)between the singlet energy EgS and the triplet energy EgT can beachieved by a small exchange interaction therebetween. Physical detailsof the relationship between ΔST and the exchange interaction areexemplarily described in the following:

-   Document 3: Organic EL Symposium, proceeding for the tenth meeting    edited by Chihaya Adachi et al., S2-5, pp. 11-12; and-   Document 4: Organic Photochemical Reaction Theory edited by Katsumi    Tokumaru, Tokyo Kagaku Dojin Co., Ltd. (1973).

Such a material can be synthesized according to molecular design basedon quantum calculation. Specifically, the material is a compound inwhich a LUMO electron orbit and a HOMO electron orbit are localized toavoid overlapping.

Examples of the compound having a small ΔST, which is used as the hostmaterial in the exemplary embodiment, are compounds in which a donorelement is bonded to an acceptor element in a molecule and ΔST is in arange of 0 eV or more and less than 0.3 eV in terms of electrochemicalstability (oxidation-reduction stability).

An aspect of examples of the donor element is a carbazole structure andan arylamine structure.

An aspect of examples of the acceptor element is an azine ringstructure, an aza-aromatic ring structure, an aza-oxygen-containing ringstructure, a CN-substituted aromatic ring and a ketone-containing ring.

In the exemplary embodiment, cyclic structures including carbazole, anazine ring, an aza-aromatic ring, and an aza-oxygen-containing ring as apartial structure are also respectively referred to as the carbazolestructure, the azine ring structure, the aza-aromatic ring structure,and the aza-oxygen-containing ring structure. The cyclic structures mayhave a substituent as needed. Examples of the substituent thereforinclude an alkyl group having 6 to 40 carbon atoms, a heterocyclic grouphaving 2 to 40 carbon atoms, a trialkylsilyl group, dialkylarylsilylgroup, an alkyldiarylsilyl group, a triarylsilyl group, a fluorine atom,and a cyano group. The trialkylsilyl group, the dialkylarylsilyl group,the alkyldiarylsilyl group, and the triarylsilyl group as thesubstituent contain at least one of an alkyl group having 1 to 30 carbonatoms and an aryl group having 6 to 30 carbon atoms. A hydrogen atomincludes a deuterium atom.

Bonding between the donor element and the acceptor element means bondingby various linking groups. An aspect of examples of the linking group isa single bond, a phenylene structure and metabiphenylene structure. Acompound having ΔST of less than 0.3 eV is usable as the host materialin the exemplary embodiment when the compound is quantum-chemicallyobserved based on the disclosure of the exemplary embodiment of theinvention and is optimized.

A more preferable compound is such a compound that dipoles formed in theexcited state of a molecule interact with each other to form anaggregate having a reduced exchange interaction energy. According toanalysis by the inventors, the dipoles are oriented substantially in thesame direction in the compound, so that ΔST can be further reduced bythe interaction of the molecules. In such a case, ΔST can be extremelysmall in a range of 0 eV to 0.2 eV.

Aggregate

Decrease in the energy gap (ΔST) between the singlet energy EgS and thetriplet energy EgT can also be achieved by aggregate formation. Herein,the aggregate does not reflect an electronic state by a single molecule,but the aggregate is provided by several molecules physicallyapproaching each other. After the plurality of molecules approach eachother, electronic states of a plurality of molecules are mixed andchanged, thereby changing an energy level. A value of singlet energy isdecreased, thereby decreasing a value of ΔST. The decrease in the valueof ΔST by the aggregate formation can also be explained by Davydovsplitting model showing that two molecules approach each other to changeelectronic states thereof (see FIG. 2). As shown in Davydov splittingmodel, it is considered that change of the electronic states by twomolecules different from change of an electronic state by a singlemolecule is brought about by two molecules physically approaching eachother. A singlet state exists in two states represented by S1−m⁺ andS1−m⁻. A triplet state exists in two states represented by T1− andT1−m⁻. Since S1−m⁻ and T1−m⁻ showing a lower energy level exist, ΔSTrepresenting a gap between S1−m⁻ and T1−m⁻ becomes smaller than that inthe electronic state by a single molecule.

The Davydov splitting model is exemplarily described in the following:

-   Document 5: J. Kang, et al, International Journal of Polymer    Science, Volume 2010, Article ID 264781;-   Document 6: M. Kasha, et al, Pure and Applied Chemistry, Vol. 11, p    371, 1965; and-   Document 7: S. Das, et al, J. Phys. Chem. B. vol. 103, p 209, 1999.

The inventors found usage of sublevels of a singlet state and a tripletstate of a compound easily forming an aggregate in a thin film, andconsequent possibility of promotion of inverse intersystem crossing bymolecules and aggregates in the thin film.

For instance, a compound having a large half bandwidth of aphotoluminescence spectrum is considered to easily form an aggregate ina thin film of the compound. A relationship between the half bandwidthof the photoluminescence spectrum and easy formability of the aggregatecan be estimated as follows.

In a compound having a property of typically existing as a singlemolecule without forming an aggregate, a vibrational level is lessrecognized in the singlet state, so that a narrow half bandwidth of thephotoluminescence spectrum is observed. For instance, CBP exhibits aproperty to typically exist as a single molecule, in which a halfbandwidth of a photoluminescence spectrum is relatively narrow as muchas about 50 nm.

On the other hand, in the compound easily forming the aggregate, aplurality of molecules electronically influence each other, whereby alot of vibrational levels exist in the singlet state. As a result, sincethe vibrational levels of the singlet state are often relaxed to theground state, the half bandwidth of the photoluminescence spectrum isincreased.

Such a compound easily forming the aggregate is expected to have a lotof vibrational levels even in a triplet state. Consequently, it isspeculated that ΔST in relation to heat is decreased through thesublevels to promote the inverse intersystem crossing, since a lot ofsublevels exist between the singlet state and the triplet state.

It should be noted that the aggregate according to the exemplaryembodiment means that a single molecule forms any aggregate with anothersingle molecule. In other words, a specific aggregate state is not shownin the exemplary embodiment. An aggregate state of an organic moleculeis probably formable in various states in a thin film, which isdifferent from an aggregate state of an inorganic molecule.

TADF Mechanism

As described above, when ΔST(H) of the organic material is small,inverse intersystem crossing from the triplet level of the host materialto the singlet level thereof is easily caused by heat energy given fromthe outside. Herein, an energy state conversion mechanism to performspin exchange from the triplet state of electrically excited excitonswithin the organic EL device to the singlet state by inverse intersystemcrossing is referred to as TADF Mechanism.

In the exemplary embodiment, since the material having a small ΔST(H) isused as the host material, inverse intersystem crossing from the tripletlevel of the host material to the singlet level thereof is easily causedby heat energy given from the outside.

FIG. 3 shows a relationship in energy level between the host materialand the dopant material in the emitting layer. In FIG. 3, S0 representsa ground state, S1_(H) represents a lowest singlet state of the hostmaterial, T1_(H) represents a lowest triplet state of the host material,S1_(D) represents a lowest singlet state of the dopant material, andT1_(D) represents a lowest triplet state of the dopant material. Asshown in FIG. 3, a difference between S1_(H) and T1_(H) corresponds toΔST(H), a difference between S1_(H) and S0 corresponds to EgS(H), adifference between S1_(D) and S0 corresponds to EgS(D) corresponds toEgS(D), and a difference between T1_(H) and T1_(D) corresponds to ΔT. Adotted-line arrow shows energy transfer between the respective excitedstates in FIG. 3.

As described above, a material having a small ΔST is selected as thecompound for the host material in the exemplary embodiment. This isbecause the material having a small ΔST(H) is considered to easily causeinverse intersystem crossing from the triplet excitons generated in thelowest triplet state T1₁₁ to the lowest singlet state S1₁₁ of the hostmaterial by heat energy. Due to the small ΔST(H), inverse intersystemcrossing is easily caused, for instance, even around a room temperature.When the inverse intersystem crossing is thus easily caused, a ratio ofenergy transfer from the host material to the lowest singlet stateT1_(D) of the fluorescent dopant material is increased by Förstertransfer, resulting in improvement in a luminous efficiency of afluorescent organic EL device.

In other words, use of the compound having a small ΔST(H) as the hostmaterial increases emission by the TADF mechanism, so that a delayedfluorescence ratio becomes large. When the delayed fluorescence ratio islarge, a high internal quantum efficiency is achievable. It isconsidered that the internal quantum efficiency can be theoreticallyraised up to 100% even by using delayed fluorescence by the TADFmechanism.

FIG. 4 shows a relationship in energy level between the host materialand the dopant material in the emitting layer in the TADF mechanismdescribed in Document 1. In FIG. 4, S0, S1_(H), T1_(H), S1_(D), andT1_(D) represent the same as those in FIG. 3. A dotted-line arrow showsenergy transfer between the respective excited states. As shown in FIG.4, a material having a small ΔST(D) is used as the dopant material inthe TADF mechanism described in Document 1. Accordingly, energy istransferred from the lowest triplet state T1_(H) of the host material tothe lowest triplet state T1_(D) of the dopant material by Dextertransfer. Further, inverse intersystem crossing from the lowest tripletstate T1_(D) to the lowest singlet state S1_(D) of the dopant materialis possible by heat energy. As a result, fluorescent emission from thelowest triplet state T1_(D) of the dopant material can be observed. Itis considered that the internal quantum efficiency can be theoreticallyraised up to 100% also by using delayed fluorescence by the TADFmechanism.

As described in Document 2, the inventors employ a fluorescent compoundhaving a small ΔST(H) in a host-dopant system.

The inventors used a fluorescent compound having a small ΔST(H) as thehost material because of the following detailed reasons.

First, considering conversion of energy states on the dopant material bythe TADF mechanism, the dopant material has a relatively high singletenergy for fluorescent emission and triplet energy approximatelyequivalent to the singlet energy. In order to efficiently trap thetriplet energy within the emitting layer, it is necessary to select ahost material having larger triplet energy. If a typical organicmaterial usually having a large ΔST is used as the host material, thesinglet energy of the host material, i.e., an energy gap between a HOMOlevel and a LUMO level becomes extremely large. As a result, an energygap between the host material and a carrier transporting layer adjacentto the emitting layer becomes large, so that injection of carriers tothe emitting layer is considered to become difficult. Accordingly, theinventors consider that conversion of the energy states by the TADFmechanism is preferably performed on the host material, whereby thecarriers are advantageously injected to the emitting layer and areeasily balanced in the entire organic EL device.

Secondly, the inventors believe it possible to suppress decrease in aluminous efficiency caused by Triplet-Triplet-Annihilation in a highcurrent density area by using the fluorescent compound having a smallΔST(H) as the host material. Herein, Triplet-Triplet-Annihilation(hereinafter, referred to as TTA) is a physical phenomenon in whichlong-life triplet excitons generated on a molecule are adjacent to eachother at a high density to collide with each other and is thermallydeactivated.

The inventors believe it possible to suppress decrease in the luminousefficiency in the high current density area to some extent in thehost-dopant system in which the triplet energy is difficult to transitfrom the host material to the dopant material. In the exemplaryembodiment, the compound having a small ΔST is used as the host materialof the emitting layer. After inverse intersystem crossing from a tripletexcited level of the host material to a singlet excited level thereof bythe TADF mechanism, energy is transferred to a singlet excited level ofthe dopant material. Accordingly, the generated triplet excitons arekept in a triplet excited state on the host material whose abundanceratio is high in the emitting layer. On the other hand, if the compoundhaving a small ΔST is used as the dopant material in the emitting layer,the generated triplet excitons are kept in a triplet excited state onthe dopant material whose abundance ratio is extremely low in theemitting layer. In other words, the inventors believe it preferable todesign a system that avoids concentration of triplet excited state onthe dopant material in driving the organic EL in the high currentdensity area. Accordingly, in the exemplary embodiment, the inventorsemploy the material having a small ΔST(H) as the host material.

Thirdly, a material having a high emission quantum efficiency can beeasily selected as the dopant material by using a material causinginverse intersystem crossing from the triplet level to the singlet levelas the host material. As a result, emission of the singlet excitons isquickly relaxed after energy transfer thereof to the dopant material, sothat energy quenching in the high current density area is suppressible.In the host-dopant system in a fluorescent device, generally, the hostmaterial has a carrier transporting function and an exciton generatingfunction and the dopant material has an emission function. This systemis for separating the carrier transporting function and the emissionfunction of the emitting layer. Accordingly, effective organic ELemission is promoted by doping a small amount of a dopant materialhaving a high emission quantum efficiency into the emitting layer. Theemitting layer according to the exemplary embodiment is required to havea function to cause inverse intersystem crossing by the TADF function inaddition to a typical function of the emitting layer. By requiring thehost material to have the function to cause inverse intersystem crossingby the TADF function, the inventors increased choices of the dopantmaterial having a high emission quantum efficiency which largelycontributes to the luminous efficiency of the organic EL. With thisarrangement, a fluorescent dopant material typically known as beinghighly efficient can be selected. Relationship between EgT and Eg_(77K)

In this exemplary embodiment, the compound having ΔST of a predeterminedvalue or less is used. The aforementioned triplet energy EgT isdifferent from a typically defined triplet energy. Such a differencewill be described below.

For general measurement of the triplet energy, a target compound to bemeasured is dissolved in a solvent to form a sample. A phosphorescentspectrum (ordinate axis: phosphorescent luminous intensity, abscissaaxis: wavelength) of the sample is measured at a low temperature (77K).A tangent is drawn to the rise of the phosphorescent spectrum on theshort-wavelength side. The triplet energy is calculated by apredetermined conversion equation based on a wavelength value at anintersection of the tangent and the abscissa axis.

As described above, the compound for the host material in the exemplaryembodiment has a small ΔST. When ΔST is small, intersystem crossing andinverse intersystem crossing are likely to occur even at a lowtemperature (77K), so that the singlet state and the triplet statecoexist. As a result, the spectrum to be measured in the same manner asthe above includes emission from both the singlet state and the tripletstate. Although it is difficult to distinguish emission from the singletstate from emission from the triplet state, the value of the tripletenergy is basically considered dominant.

Accordingly, in order to distinguish the triplet energy EgT in theexemplary embodiment from the typical triplet energy EgT in a strictmeaning although the measurement method is the same, the triplet energyEgT in the exemplary embodiment is defined as follows. A target compoundto be measured is dissolved in a solvent to form a sample. Aphosphorescent spectrum (ordinate axis: phosphorescent luminousintensity, abscissa axis: wavelength) of the sample is measured at a lowtemperature (77K). A tangent is drawn to the rise of the phosphorescentspectrum on the short-wavelength side. Energy is calculated as an energygap Eg_(77K) by a predetermined conversion equation based on awavelength value at an intersection of the tangent and the abscissaaxis. ΔST is defined as a difference between the singlet energy EgS andthe energy gap Eg_(77K). Accordingly, ΔST(H) is represented by theformula (1).

The triplet energy measured in a solution state may include an error byinteraction between the target molecule and the solvent. Accordingly, asan ideal condition, a measurement in a thin film state is desired inorder to avoid the interaction between the target molecule and thesolvent. In this exemplary embodiment, the molecule of the compound usedas the host material exhibits a photoluminescence spectrum having abroad half bandwidth in a solution state, which strongly impliesaggregate formation also in the solution state. Accordingly, thesolution state is considered to be under the same conditions as in athin film state. Consequently, in this exemplary embodiment, ameasurement value of the triplet energy in the solution state is used.

Singlet Energy EgS

The singlet energy EgS in the exemplary embodiment is defined based oncalculation by a typical method. Specifically, the target compound isevaporated on a quartz substrate to prepare a sample. An absorptionspectrum (ordinate axis: absorbance, abscissa axis: wavelength) of thesample is measured at a normal temperature (300K). A tangent is drawn tothe rise of the absorption spectrum on the long-wavelength side. Thesinglet energy EgS is calculated by a predetermined conversion equationbased on the tangent and the wavelength value at the intersection. EgSin aggregate formation corresponds to an energy gap between S1−m− andthe ground state S0 in the Davydov splitting model.

The calculation of the singlet energy EgS and the energy gap Eg_(77K)will be described in detail later.

Delayed Fluorescence Ratio

It was found that a delayed fluorescence ratio according to the organicEL device of the exemplary embodiment exceeds the theoreticalupper-limit of a delayed fluorescence ratio (TTF ratio) of a case whereit is assumed that delayed fluorescence is generated only by the TTFmechanism. In other words, according to the exemplary embodiment, anorganic EL device having a higher internal quantum efficiency isachievable.

The delayed fluorescence ratio is measurable by a transitional ELmethod. The transitional EL method is for measuring reduction behavior(transitional property) of EL emission after pulse voltage applied onthe device is removed. EL luminous intensity is classified into aluminescence component from singlet excitons generated in firstrecombination and a luminescence component from singlet excitonsgenerated through triplet excitons. Since lifetime of the singletexcitons generated in the first recombination is very short at anano-second order, EL emission is rapidly reduced after removal of pulsevoltage.

On the other hand, since delayed fluorescence provides emission fromsinglet excitons generated through long-life triplet excitons, ELemission is gradually reduced. Thus, since there is a large differencein time between emission from the singlet excitons generated in thefirst recombination and emission from the singlet excitons derived fromthe triplet excitons, a luminous intensity derived from delayedfluorescence is obtainable. Specifically, the luminous intensity can bedetermined by the following method.

Transitional EL waveform is measured as follows (see FIG. 5). Pulsevoltage waveform outputted from a voltage pulse generator (PG) 11 isapplied on an organic EL device (EL) 12. The applied voltage waveform isloaded in an oscilloscope (OSC) 13. When pulse voltage is applied on theorganic EL device 12, the organic EL device 12 generates pulse emission.This emission is loaded in the oscilloscope (OSC) 13 through aphotomultiplier (PMT) 14. The voltage waveform and the pulse emissionare synchronized and loaded in a personal computer (PC) 15.

The ratio of luminous intensity derived from delayed fluorescence isdefined as follows based on analysis of the transitional EL waveform. Itshould be noted that a formula to calculate a TTF ratio described inInternational Publication No. WO2010/134352 may be used for calculationof the ratio of luminous intensity derived from delayed fluorescence.

It is considered that a delayed fluorescence component defined in theexemplary embodiment includes thermally activated delayed fluorescence(TADF mechanism) recited in the exemplary embodiment in addition to theluminescence component derived from TTF. For this reason, in theexemplary embodiment, a ratio of the delayed fluorescence componentcalculated according to the following formula (4) is referred to as adelayed fluorescence ratio, not as a TTF ratio.

The delayed fluorescence ratio is calculated according to the formula(4).

$\begin{matrix}{\frac{1}{\sqrt{I}} \propto {A + {\gamma \cdot t}}} & (4)\end{matrix}$

In the formula (4), I represents luminous intensity derived from delayedfluorescence. A represents a constant. The measured transitional ELwaveform data is fit in the formula (4) to obtain the constant A. Here,a luminous intensity 1/A² at the time t=0 when pulse voltage is removedis defined as the ratio of luminous intensity derived from delayedfluorescence.

A graph of FIG. 6A shows a measurement example where a predeterminedpulse voltage is applied on the organic EL device and then the pulsevoltage is removed and shows time-varying luminous intensities of theorganic EL device.

The pulse voltage was removed at the time of about 3×10⁻⁸ seconds in thegraph of FIG. 6A. In the graph of FIG. 6A, the luminous intensity whenthe voltage is removed is defined as 1.

After rapid reduction before the elapse of about 2×10⁻⁷ seconds afterthe voltage removal, a gradual reduction component appears.

In the graph of FIG. 6B, the voltage removal time is a starting pointand the inverse square root of luminous intensity before the elapse of1.5×10⁻⁵ seconds after voltage removal is plotted. Fitting is conductedas follows.

A value at an intersection A of the ordinate axis and the linear lineextended to the starting point is 1.55. Accordingly, the ratio ofluminous intensity derived from the delayed fluorescence obtained fromthe transitional EL waveform is 1/(1.55)²=0.41, which means 41% of theluminous intensity was derived from the delayed fluorescence. In otherwords, the ratio of luminous intensity exceeds 37.5%, i.e., the supposedtheoretical upper-limit of the TTF ratio.

The luminous intensity derived from the delayed fluorescence obtainedfrom the transitional EL waveform is variable in accordance withmeasurement temperatures. Such a phenomenon is considered to be inherentmostly in fluorescent emission by the TADF mechanism

The luminous intensity is preferably fitted in a linear line by themethod of least squares. In this case, the luminous intensity before theelapse of 10⁻⁵ seconds is preferably fitted.

The TTF mechanism having an emission mechanism by delayed fluorescencewill be described using FIG. 7. FIG. 7 shows a relationship in energylevel between the host material and the dopant material in an organic ELdevice using the TTF mechanism. In FIG. 7, S0, S1_(H), T1_(H), S1_(D)and T1_(D) represent the same as those in FIG. 3. An arrow shows energytransfer between the respective excited states in FIG. 7.

As described above, the TTF mechanism utilizes a phenomenon in whichsinglet excitons are generated by collision between two tripletexcitons. As shown in FIG. 7, it is preferable that the lowest tripletstate T1_(H) of the host material is lower than the lowest triplet stateT1_(D) of the dopant material, so that triplet excitons concentrate onmolecules of the host material. The triplet excitons efficiently collidewith each other in accordance with increase in the density of thetriplet excitons, whereby the triplet excitons are partially changedinto singlet excitons. The lowest singlet state S1_(H) of the hostmaterial generated by the TTF mechanism is immediately transferred tothe lowest singlet state S1_(D) of the dopant material by Förstertransfer, so that the dopant material emits fluorescence.

The theoretical upper-limit of the TTF ratio can be obtained as follows.

According to S. M. Bachilo et al. (J. Phys. Chem. A, 104, 7711 (2000)),assuming that high-order excitons such as quintet excitons are quicklyreturned to triplet excitons, triplet excitons (hereinafter abbreviatedas ³A*) collide with one another when the density thereof is increased,whereby a reaction shown by the following formula (5) occurs. In theformula, ¹A represents the ground state and ¹A* represents the lowestsinglet excitons.

³ A*+ ³ A*→(4/9)¹ A+1/9)¹ A*(13/9)³ A*  (1)

In short,

5³ A*→4¹ A+ ¹ A*

It is expected that, among triplet excitons initially generated, whichaccount for 75%, one fifth thereof (i.e., 20%) is changed to singletexcitons.

Accordingly, the amount of singlet excitons which contribute to emissionis 40%, which is a value obtained by adding 15% (75%×(1/5)=15%) to 25%,which is the amount ratio of initially generated singlet excitons.

At this time, a ratio of luminous intensity derived from TTF (TTF ratio)relative to the total luminous intensity is 15/40, i.e., 37.5%. Thus, itis recognized that the delayed fluorescence ratio of the organic ELdevice according to the exemplary embodiment exceeds the theoreticalupper-limit of only the TTF ratio.

Residual Intensity Ratio in 1 μs

A method for relatively measuring an amount of delayed fluorescence isexemplified by a method for measuring a residual intensity in 1 μs. Theresidual intensity in 1 μs is defined as a ratio of a luminous intensityafter the elapse of 1 μs after removal of a pulse voltage measured by atransitional EL method to a voltage at the time of the removal of thepulse voltage. The relative amount of delayed fluorescence can beestimated based on reduction behavior of EL emission after the removalof the pulse voltage measured by the transitional EL method. Theresidual intensity ratio in 1 μs can be obtained by reading luminousintensity at the time of 1.0 μs in the graph of FIG. 6A.

The residual intensity ratio in 1 μs is preferably larger than 36.0%,more preferably 38.0 or more.

Dopant Properties

A preferable dopant in the exemplary embodiment has properties to emitfluorescence and to have a large speed constant of radiationaltransition. In this arrangement, singlet excitons electrically excitedon the host material, singlet excitons generated by the TADF mechanismand the like are transferred to singlet excitons of the dopant materialby Förster energy transfer and the dopant material immediately emitslight. In other words, fluorescent emission is possible through theabove energy transition before triplet excitons on the host materialcauses TTA, by which decrease in an efficiency in the high current areais likely to be considerably improved.

It is preferable to select a dopant material having a fluorescencelifetime of 5 ns or less, more preferably 2 ns or less as the dopantmaterial having a large speed constant of radiational transition in theexemplary embodiment. A fluorescence quantum efficiency of the dopantmaterial is preferably 80% or more in a solution. The fluorescencequantum efficiency can be obtained by measuring the dopant material in arange of 10⁻⁵ to 10⁻⁶ mol/l of a concentration in a toluene solutionusing Absolute PL Quantum Yield Measurement System C9920-02 manufacturedby HAMAMATSU PHOTONICS K.K.

It is also expected by measuring an EL spectrum of the device andconfirming a luminescence component of a material other than the dopantmaterial is 1/10 or less of the luminescence component of the dopantthat the dopant material has a large speed constant of radiationaltransition.

Relationship Between Emitting Layer and Electron Transporting Layer

When ΔST(H) of the host material is small, the energy gap between thehost material and the electron transporting layer adjacent thereto issmall, so that the electrons are likely to be injected into the emittinglayer. As a result, carrier balance is easily obtainable to decreaseroll-off.

Relationship Between Emitting Layer and Hole Transporting Layer

When an ionization potential of the hole transporting layer isrepresented by IP_(HT), IP_(HT)≦5.7 eV is preferable. With thisarrangement, balance between the electrons and the holes can beenhanced. The ionization potential can be obtained, for instance, bymeasuring the material in a form of a thin film using a photoelectronspectroscopy (AC-3: manufactured by RIKEN KEIKI Co., Ltd.).

Relationship in Singlet Energy Between Host Material and Dopant Material

In the exemplary embodiment, the dopant material is a fluorescent dopantmaterial. A compound used as the host material and a compound used asthe dopant material satisfy a relationship represented by the formula(2) in terms of the singlet energy. When such a relationship issatisfied, energy of the singlet excitons initially generated on thehost material and the singlet excitons derived from the delayedfluorescence is easily transferred to the dopant material. Consequently,the dopant efficiently emits fluorescence.

Δn

The inventors found that one way to reduce ΔST is to use the compoundforming the aggregate and that the compound having a large Δn easilyforms the aggregate in a film of the compound. Herein, Δn is a valuerepresenting the largest difference between the refractive index n_(Z)perpendicular to the silicon substrate surface and the refractive indexn_(X) parallel to the silicon substrate surface in an area where areflectivity to be observed simultaneously with a refractivity is notobserved, in the spectroscopic ellipsometry measurement (measurementrange: 200 nm to 1000 nm).

A relationship between Δn and easy formability of the aggregate isestimated as follows.

When a large difference is generated between a refractive index n in avertical direction z relative to the silicon substrate and a refractiveindex n in a parallel direction x relative to the silicon substrate, itis considered that molecules exist with a certain regularity in a thinfilm state. In other words, the compound used as the host material inthe exemplary embodiment is expected to have a predetermined value of Δnwhile forming the aggregate in the thin film state to exhibit a certainregularity.

On the other hand, a compound having an extremely small Δn (e.g., CBPand Alq₃) exists in an amorphous state in which molecules have noregularity in a thin film state.

The relationship between Δn and easy formability of the aggregate isdescribed in the following:

-   Document 8: D. Yokoyama et al., Org. Electron. 10, 127-137 (2009);-   Document 9: D. Yokoyama et al., Appl. Phys. Lett. 93, 173302 (2008);    and-   Document 10: D. Yokoyama et al., Appl. Phys. Lett. 95, 243303    (2009).

An can be calculated based on the refractive index of each compoundmeasured by the spectroscopic ellipsometry method. The spectroscopicellipsometry method is a measurement method of an optical constant(i.e., a refractive index n and an extinction coefficient k) and athickness of a thin film. For instance, a variable-incident-anglehigh-speed spectroscopic ellipsometer (M-2000D: manufactured by J. A.Woollam Co., Inc.) is usable. FIGS. 8A and 8B show an example ofspectroscopic ellipsometry measurement. FIG. 8A shows an incident angleof an incident light from a light source. FIG. 8B shows a cross sectionof an organic thin film (a measurement target) on a silicon substrate.

Each compound is evaporated on the silicon substrate (Si (100)) to forma 100-nm organic thin film. Using the variable-incident-angle high-speedspectroscopic ellipsometer (M-2000D: manufactured by J. A. Woollam Co.,Inc.), ellipsometric parameters ψ and Δ are measured at every fivedegrees in a range of 45 degrees to 80 degrees of an incident angle andat every 1.6 nm in a range of 200 nm to 1000 nm of a wavelength. Theobtained parameters are analyzed together using an analysis softwareWVASE32 (manufactured by J. A. Woollam Co., Inc) to examine opticalanisotropy of the film. The anisotropy of the optical constant (i.e.,the refractive index n and the extinction coefficient k) of the filmreflects the anisotropy of molecular orientation in the film. Themeasurement method and the analysis methods are described in detail inthe above Documents 8 to 10.

Δn can be obtained as a difference between the refractive index n in theperpendicular direction z relative to the silicon substrate and therefractive index n in the parallel direction x relative to the siliconsubstrate. The perpendicular direction z and the parallel direction xrelative to the silicon substrate are shown in FIG. 8A.

Half Bandwidth

A half bandwidth represents a width of an emission spectrum when aluminous intensity becomes half relative to the maximum luminousintensity of the emission spectrum. The inventors found that a hostmaterial having 50 nm or more of a half bandwidth of a photoluminescencespectrum is a material easily forming an aggregate state and easilycausing inverse intersystem crossing in a thin film. Accordingly, theTADF mechanism easily works in the host material having 50 nm or more ofthe half bandwidth of the photoluminescence spectrum. Particularlypreferably, the half bandwidth of the photoluminescence spectrum of thehost material is 75 nm or more.

ΔT

It is preferable that a difference ΔT between triplet energy Eg_(77K)(H)of the host material and triplet energy Eg_(77K)(D) of the dopantmaterial satisfies a relationship represented by the formula (3). ΔT ismore preferably 0.8 eV or more, further preferably 1.0 eV or more.

When ΔT satisfies the relationship represented by the formula (3),energy of the triplet excitons generated by recombination on the hostmaterial becomes difficult to transfer to the triplet level of thedopant material, and thermal deactivation of the triplet excitonsbecomes difficult. Consequently, the dopant efficiently emitsfluorescence.

Compound(s) of Emitting Layer

Compounds satisfying the relationships represented by the formulae (1)and (2) and used as the host material and the dopant material are asfollows.

Host Material

Examples of the host material include a carbazole derivative, abiscarbazole derivative, an indolocarbazole derivative, an acridinederivative, an oxazine derivative, a pyrazine derivative, a pyrimidinederivative, a triazine derivative, a dibenzofuran derivative, and adibenzothiophene derivative. These derivatives may have a substituent asneeded.

Examples of the substituent therefor include an alkyl group having 6 to40 carbon atoms, a heterocyclic group having 2 to 40 carbon atoms, atrialkylsilyl group, dialkylarylsilyl group, an alkyldiarylsilyl group,a triarylsilyl group, a fluorine atom, and a cyano group. Thetrialkylsilyl group, the dialkylarylsilyl group, the alkyldiarylsilylgroup, and the triarylsilyl group as the substituent contain at leastone of an alkyl group having 1 to 30 carbon atoms and an aryl grouphaving 6 to 30 carbon atoms. A hydrogen atom includes a deuterium atom.

The host material is preferably a compound including bonding between atleast one selected from a carbazole structure, a biscarbazole structure,an indolocarbazole structure, and an acridine structure and at least oneselected from an oxazine structure, a pyrazine structure, a pyrimidinestructure, a triazine structure, and a dibenzofuran structure.

Bonding between these structures means bonding by various linkinggroups. An aspect of examples of the linking group is a single bond, aphenylene structure and metabiphenylene structure.

In the exemplary embodiment, the carbazole structure, theindolocarbazole structure, the acridine structure, the oxadinestructure, the pyrazine structure, the pyrimidine structure, thetriazine structure, and the dibenzofuran structure respectively refer tocyclic structures containing indolocarbazole, acridine, oxadine,pyrazine, pyrimidine, triazine, and dibenzofuran as a partial structure.

The carbazole structure, the biscarbazole structure, the indolocarbazolestructure, the acridine structure, the oxazine structure, the pyrazinestructure, the pyrimidine structure, the triazine structure, and thedibenzofuran structure may have a substituent as needed.

Examples of the substituent therefor include an alkyl group having 6 to40 carbon atoms, a heterocyclic group having 2 to 40 carbon atoms, atrialkylsilyl group, dialkylarylsilyl group, an alkyldiarylsilyl group,a triarylsilyl group, a fluorine atom, and a cyano group. Thetrialkylsilyl group, the dialkylarylsilyl group, the alkyldiarylsilylgroup, and the triarylsilyl group as the substituent contain at leastone of an alkyl group having 1 to 30 carbon atoms and an aryl grouphaving 6 to 30 carbon atoms. A hydrogen atom includes a deuterium atom.

Since the most material is a compound in which a donor element is bondedto an acceptor element in a molecule, the host material is preferablyselected from compounds represented by the following formulae (101) and(102).

In the formula (101): rings A, B, and C each are a substituted orunsubstituted five- to seven-membered ring including as a ring-formingatom an atom selected from a carbon atom, a nitrogen atom, an oxygenatom, a sulfur atom, and a silicon atom; the ring A is fused with thering B and the ring C is fused with the ring B; the ring C may be fusedwith an additional ring; Q represents a group represented by a formula(103) below; and k is 1 or 2.

In the formula (103): at least one of Y¹ to Y⁶ is a carbon atom to bebonded to L; one to three of Y¹ to Y⁶ are a nitrogen atom(s); the restof Y¹ to Y⁶ of the carbon atom bonded with L or the nitrogen atom isCAr¹; Ar¹ is a substituted or unsubstituted aromatic hydrocarbon group;when the formula (103) include a plurality of CAr¹, Ar¹ is mutually thesame or different; and L represents a single bond or a linking group.

In the formula (102): the ring A, the ring B, the ring C, Q, and krepresent the same as those in the formula (101); and Ar is asubstituted or unsubstituted aromatic hydrocarbon group.

Compounds in which the ring C in the formulae (101) is fused with theadditional ring are respectively represented by the following formulae(101A) and (101B). Compounds in which the ring C in the formulae (102)is fused with the additional ring are respectively represented by thefollowing formulae (102A) and (102B).

In the each formula: the ring A, the ring B, the ring C, Ar, and Qrepresent the same as those in the formula (101); k is 1 or 2; and ringsD and E each are a substituted or unsubstituted five- to seven-memberedring including as a ring-forming atom an atom selected from a carbonatom, a nitrogen atom, an oxygen atom, a sulfur atom, and a siliconatom.

The compound represented by the formula (101) is preferably compoundsrepresented by the following formulae.

In the formulae: R represents an alkyl group; X represents CH, CRx or anitrogen atom; Rx represents a substituent; and Bx represents a five- toseven-membered ring formed of carbon atoms.

The compound represented by the formula (101) is further preferablycompounds represented by the following formulae.

The compound represented by the formula (102) is preferably compoundsrepresented by the following formulae.

In the formulae: R represents an alkyl group; X and X¹ to X⁴ representCH, CRx or a nitrogen atom; Rx represents a substituent; one of X¹ to X⁴is a carbon atom bonded to Q; Bx represents a five- to seven-memberedring formed in a carbon atom; Ar represents an aromatic hydrocarbongroup; and Ph represents a phenyl group.

Among the compounds represented by the formulae, X¹ or X³ is preferablya carbon atom bonded to Q.

The compound represented by the formula (102) is further preferablycompounds represented by the following formulae.

A group represented by the formula (103) is preferably groupsrepresented by the following formulae.

In the formulae: Ph represents a phenyl group.

Examples of the compound used as the host material in the exemplaryembodiment are shown below. However, the host material in the exemplaryembodiment is not limited thereto.

Dopant Material

In this exemplary embodiment, the fluorescent dopant material is used asthe dopant material of the emitting layer as described above.

Known fluorescent materials are usable as the fluorescent dopantmaterial. Examples of the fluorescent dopant material include abisarylamino naphthalene derivative, an aryl-substituted naphthalenederivative, a bisarylamino anthracene derivative, an aryl-substitutedanthracene derivative, a bisarylamino pyrene derivative, anaryl-substituted pyrene derivative, a bisarylamino chrysene derivative,an aryl-substituted chrysene derivative, a bisarylamino fluoranthenederivative, an aryl-substituted fluoranthene derivative, anindenoperylene derivative, a pyrromethene boron complex compound, acompound having a pyrromethene skeleton or a metal complex thereof, adiketopyrrolopyrrole derivative, and a perylene derivative.

A thickness of the emitting layer is preferably in the range of 5 nm to50 nm, more preferably in the range of 7 nm to 50 nm and most preferablyin the range of 10 nm to 50 nm. The thickness of less than 5 nm maycause difficulty in forming the emitting layer and in controllingchromaticity, while the thickness of more than 50 nm may raise drivevoltage.

In the emitting layer, a ratio of the host material and the fluorescentdopant material is preferably in a range of 99:1 to 50:50 at a massratio.

Substrate

The organic EL device according to the aspect of the invention is formedon a light-transmissive substrate. The light-transmissive substratesupports an anode, an organic compound layer, a cathode and the like ofthe organic EL device. The light-transmissive substrate is preferably asmoothly-shaped substrate that transmits 50% or more of light in avisible region of 400 nm to 700 nm.

The light-transmissive plate is exemplarily a glass plate, a polymerplate or the like.

The glass plate is formed of soda-lime glass,barium/strontium-containing glass, lead glass, aluminosilicate glass,borosilicate glass, barium borosilicate glass, quartz and the like.

The polymer plate is formed of polycarbonate, acryl, polyethyleneterephthalate, polyether sulfide and polysulfone.

Anode and Cathode

The anode of the organic EL device injects holes into the emittinglayer, so that it is efficient that the anode has a work function of 4.5eV or higher.

Exemplary materials for the anode are indium-tin oxide (ITO), tin oxide(NESA), indium zinc oxide, gold, silver, platinum and copper.

When light from the emitting layer is to be emitted through the anode,the anode preferably transmits more than 10% of the light in the visibleregion. Sheet resistance of the anode is preferably several hundredsΩ/sq. or lower. The thickness of the anode is typically in the range of10 nm to 1 μm, and preferably in the range of 10 nm to 200 nm, though itdepends on the material of the anode.

The cathode is preferably formed of a material with smaller workfunction in order to inject electrons into the emitting layer.

Although a material for the cathode is subject to no specificlimitation, examples of the material are indium, aluminum, magnesium,alloy of magnesium and indium, alloy of magnesium and aluminum, alloy ofaluminum and lithium, alloy of aluminum, scandium and lithium, and alloyof magnesium and silver.

Like the anode, the cathode may be made by forming a thin film on, forinstance, the electron transporting layer and the electron injectinglayer by a method such as vapor deposition. In addition, the light fromthe emitting layer may be emitted through the cathode. When light fromthe emitting layer is to be emitted through the cathode, the cathodepreferably transmits more than 10% of the light in the visible region.

Sheet resistance of the cathode is preferably several hundreds Ω/sq. orlower. The thickness of the cathode is typically in the range of 10 nmto 1 μm, and preferably in the range of 50 nm to 200 nm, though itdepends on the material of the cathode.

Hole Injecting/Transporting Layer

The hole injection/transport layer helps injection of holes to theemitting layer and transport the holes to an emitting region. A compoundhaving a large hole mobility and a small ionization energy is used asthe hole injection/transport layer.

A material for forming the hole injection/transport layer is preferablya material of transporting the holes to the emitting layer at a lowerelectric field intensity. For instance, an aromatic amine compound ispreferably used.

Electron Injecting/Transporting Layer

The electron injecting/transporting layer helps injection of theelectrons into the emitting layer and transports the electrons to anemitting region. A compound having a large electron mobility is used asthe electron injecting/transporting layer.

A preferable example of the compound used as the electroninjecting/transporting layer is an aromatic heterocyclic compound havingat least one heteroatom in a molecule. Particularly, anitrogen-containing cyclic derivative is preferable. Thenitrogen-containing cyclic derivative is preferably a heterocycliccompound having a nitrogen-containing six-membered or five-membered ringskeleton.

In the organic EL device in the exemplary embodiment, in addition to theabove exemplary compound, any compound selected from compounds known asbeing used in the typical organic El device is usable as a compound forthe organic compound layer other than the emitting layer.

Layer Formation Method(s)

A method for forming each layer of the organic EL device in theexemplary embodiment is subject to no limitation except for the aboveparticular description.

However, known methods of dry film-forming such as vacuum deposition,sputtering, plasma or ion plating and wet film-forming such as spincoating, dipping, flow coating or ink-jet are applicable.

Thickness

The thickness of each organic layer of the organic EL device in theexemplary embodiment is subject to no limitation except for thethickness particularly described above. However, the thickness istypically preferably in a range of several nanometers to 1 μm because anexcessively thin film is likely to entail defects such as a pin holewhile an excessively thick film requires high applied voltage anddeteriorates efficiency.

Modifications of Exemplary Embodiment

It should be noted that the invention is not limited to the aboveexemplary embodiment but may include any modification and improvement aslong as such modification and improvement are compatible with theinvention.

The emitting layer is not limited to a single layer, but may be providedas laminate by a plurality of emitting layers. When the organic ELdevice includes the plurality of emitting layers, it is only requiredthat at least one of the emitting layers includes the host material andthe fluorescent dopant material defined in the exemplary embodiment. Theothers of the emitting layers may be a fluorescent emitting layer or aphosphorescent emitting layer.

When the organic EL device includes the plurality of emitting layers,the plurality of emitting layers may be adjacent to each other.

Further, the materials and treatments for practicing the invention maybe altered to other arrangements and treatments as long as such otherarrangements and treatments are compatible with the invention.

EXAMPLES

Examples of the invention will be described below. However, theinvention is not limited by these Examples.

Used compounds are as follows.

Synthesis of Compound(s) Synthesis Example 1 (Synthesis of GH-4)

Under an argon gas atmosphere, an intermediate A (4.4 g, 21 mmol)synthesized according to the method described in JP-A-2010-180204, anintermediate B (4.7 g, 10 mmol) synthesized according to the methoddescribed in International Publication No. WO03/080760,tris(dibenzylidene acetone)dipalladium (0.37 g, 0.4 mmol),tri-t-butylphosphonium tetrafluoroborate (0.46 g, 1.6 mmol),t-butoxysodium (2.7 g, 28 mmol) and anhydrous toluene (100 ml) weresequentially added and refluxed for eight hours.

After the reaction solution was cooled down to the room temperature, anorganic layer was separated and an organic solvent was distilled awayunder reduced pressure. The obtained residue was refined by silica-gelcolumn chromatography, so that a target compound GH-4 (3.6 g, a yield of50%) was obtained.

FD-MS analysis consequently showed that m/e was equal to 722 while acalculated molecular weight was 722.

A synthesis scheme of the target compound GH-4 is shown below.

Synthesis Example 2 (Synthesis of BH-1)

Under a nitrogen gas atmosphere, to a flask, 3,6-dibromocarbazole (5 g,15.4 mmol), phenylboronic acid (4.1 g, 33.9 mmol),tetrakis(triphenylphosphine)palladium (0.7 g, 0.6 mmol), toluene (45 ml)and 2M sodium carbonate (45 ml) were mixed in sequence, and were stirredfor eight hours at 80 degrees C. An organic phase was separated and thenconcentrated under reduced pressure by an evaporator. The obtainedresidue thereof was refined by silica-gel column chromatography, so that3.6-diphenylcarbazole (3.6 g, a yield of 74%) was obtained.

Under an argon gas atmosphere, 2,6-dichloropyrazine (0.6 g, 3.9 mmol),3,6-dibromocarbazole (2.6 g, 8 mmol),tris(dibenzylideneacetone)dipalladium (0.07 g, 0.08 mmol),tri-t-butylphosphonium tetrafluoroborate (0.09 g, 0.3 mmol), sodiumt-butoxide (0.5 g, 5.5 mmol), and anhydrous toluene (20 ml) were mixedin sequence, and heated to reflux for 8 hours.

After the reaction solution was cooled down to the room temperature, anorganic layer was removed and an organic solvent was distilled awayunder reduced pressure. The obtained residue thereof was refined bysilica-gel column chromatography, so that 1.8 g of a solid was obtained.

FD-MS analysis consequently showed that the obtained compound wasidentified as a compound BH-1.

FD-MS: calcd for C₅₂H₃₄N₄=714, found m/z=714 (M+, 100).

Evaluation of Compounds

Next, properties of the compounds used in Example were measured. Thetarget compounds are GH-4, GD-1, BH-1, and BD-1. A measurement method ora calculation method is described below. Measurement results orcalculation results are shown in Table 1.

(1) Singlet Energy EgS

Singlet Energy EgS was obtained according to the following method.

The target compound to be measured was evaporated on a quartz substrateto prepare a sample. An absorption spectrum of the sample was measuredat a normal temperature (300K). A sample was 100 nm thick. Theabsorption spectrum was expressed in coordinates of which ordinate axisindicated absorbance and of which abscissa axis indicated thewavelength. A tangent was drawn to the fall of the absorption spectrumon the long-wavelength side, and a wavelength value λedge (nm) at anintersection of the tangent and the abscissa axis was obtained. Thewavelength value was converted to an energy value by the followingconversion equation. The energy value was defined as EgS.

The conversion equation: EgS (eV)=1239.85/λedge

For the measurement of the absorption spectrum, a spectrophotometer(U3310 manufactured by Hitachi, Ltd.) was used.

The tangent to the fall of the absorption spectrum on thelong-wavelength side was drawn as follows. While moving on a curve ofthe absorption spectrum from the maximum spectral value closest to thelong-wavelength side in a long-wavelength direction, a tangent at eachpoint on the curve is checked. An inclination of the tangent isdecreased and increased in a repeated manner as the curve falls (i.e., avalue of the ordinate axis is decreased). A tangent drawn at a point ofthe minimum inclination closest to the long-wavelength side (except whenabsorbance is 0.1 or less) is defined as the tangent to the fall of theabsorption spectrum on the long-wavelength side.

The maximum absorbance of 0.2 or less was not included in theabove-mentioned maximum absorbance on the long-wavelength side.

(2) Energy Gap Eg_(77K) and Triplet Energy EgT_(D)

Eg_(77K) and EgT_(D) were obtained by the following method.

Each of the compounds was measured by a known method of measuringphosphorescence (e.g. a method described in “Hikarikagaku no Sekai (TheWorld of Photochemistry)” (edited by The Chemical Society of Japan,1993, on and near page 50). Specifically, each compound was dissolved ina solvent (EPA (diethylether:isopentane:ethanol=5:5:5 (volume ratio), aspectral grade solvent) to provide a sample for phosphorescencemeasurement (Sample 10 μmol/liter). The sample for phosphorescencemeasurement was put into a quartz cell, cooled to 77K and irradiatedwith excitation light, so that phosphorescence intensity was measuredwhile changing a wavelength. The phosphorescence spectrum was expressedin coordinates of which ordinate axis indicated phosphorescenceintensity and of which abscissa axis indicated the wavelength.

A tangent was drawn to the rise of the phosphorescent spectrum on theshort-wavelength side, and a wavelength value λedge (nm) at anintersection of the tangent and the abscissa axis was obtained. Thewavelength value was converted to an energy value by the followingconversion equation. The energy value was defined as Eg_(77K)(H) orEgT_(D) (Eg_(77K)(D)).

$\begin{matrix}{{{The}\mspace{14mu} {conversion}\mspace{14mu} {equation}\text{:}{~~~}E\; {g_{77K}(H)}({eV})} = {1239.85/{\lambda edge}}} \\{{\text{:}{~~~}{{EgT}_{D}({eV})}} = {1239.85/{\lambda edge}}}\end{matrix}$

The tangent to the rise of the phosphorescence spectrum on theshort-wavelength side was drawn as follows. While moving on a curve ofthe phosphorescence spectrum from the short-wavelength side to themaximum spectral value closest to the short-wavelength side among themaximum spectral values, a tangent is checked at each point on the curvetoward the long-wavelength of the phosphorescence spectrum. Aninclination of the tangent is increased as the curve rises (i.e., avalue of the ordinate axis is increased). A tangent drawn at a point ofthe maximum inclination was defined as the tangent to the rise of thephosphorescence spectrum on the short-wavelength side.

The maximum with peak intensity being 10% or less of the maximum peakintensity of the spectrum is not included in the above-mentioned maximumclosest to the short-wavelength side of the spectrum. The tangent drawnat a point of the maximum spectral value being closest to theshort-wavelength side and having the maximum inclination is defined as atangent to the rise of the phosphorescence spectrum on theshort-wavelength side.

For phosphorescence measurement, a spectrophotofluorometer body F-4500and optional accessories for low temperature measurement (which weremanufactured by Hitach High-Technologies Corporation) were used. Themeasurement instrument is not limited to this arrangement. A combinationof a cooling unit, a low temperature container, an excitation lightsource and a light-receiving unit may be used for measurement.

(3) ΔST

ΔST was obtained as a difference between EgS and Eg_(77K) measured inthe above (1) and (2) (see the above formula (2)). The results are shownin Table 1.

(4) ΔT

ΔT was obtained as a difference between Eg_(77K)(H) and EgT(D) measuredin the above (1) and (2).

ΔT=Eg_(77K)(H)−EgT(D)

In a combination of the host material GH-4 and the dopant material GD-1,the following formula was satisfied.

ΔT=1.11 (eV)

It should be noted that ΔT was not obtained because EgT(D) of the dopantmaterial BD-1 was not measured in a combination of the host materialBH-1 and the dopant material BD-1.

The dopant material GD-1 was measured in a range of 10⁻⁵ to 10⁻⁶ mol/lof a concentration in a toluene solution using Absolute PL Quantum YieldMeasurement System C9920-02 manufactured by HAMAMATSU PHOTONICS K.K. Asa result, an absolute PL quantum yield was 100%.

The dopant material BD-1 was measured in a range of 10⁻⁵ to 10⁻⁶ mol/lof a concentration in a toluene solution using Absolute PL Quantum YieldMeasurement System C9920-02 manufactured by HAMAMATSU PHOTONICS K.K. Asa result, an absolute PL quantum yield was 90%.

A compound HT-1 in a form of a thin film was measured in terms ofionization potential (also referred to as IP) using a photoelectronspectroscopy (AC-3: manufactured by RIKEN KEIKI Co., Ltd.). As a result,IP was 5.6 eV.

A half bandwidth of photoluminescence spectrum was obtained as follows.

Each compound was dissolved in a solvent (dichloromethane) to prepare asample for fluorescence measurement (Sample 10 μmol/liter). The samplefor fluorescence measurement was put into a quartz cell and irradiatedwith excitation light at a normal temperature (300K), so thatfluorescence intensity was measured while changing a wavelength. Thephotoluminescence spectrum was expressed in coordinates of whichordinate axis indicated fluorescence intensity and of which abscissaaxis indicated the wavelength. For fluorescence measurement, aspectrophotofluorometer F-4500 (manufactured by Hitach High-TechnologiesCorporation) was used.

The half bandwidth (unit: nm) was measured based on thephotoluminescence spectrum.

The compounds GH-4 and BH-1 were measured with respect to the halfbandwidth. As a result, the half bandwidth of the compounds GH-4 andBH-1 were respectively 79 nm and 98 nm.

TABLE 1 EgS Eg77K Δ ST [eV] [eV] [eV] GH-4 2.98 2.91 0.07 GD-1 2.47 1.800.67 BH-1 2.90 2.84 0.06 BD-1 2.69 — —

Preparation and Evaluation of Organic EL Device

The organic EL device was prepared and evaluated as follows.

Example 1

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured byGeomatec Co., Ltd.) having an ITO transparent electrode (anode) wasultrasonic-cleaned in isopropyl alcohol for five minutes, and thenUV/ozone-cleaned for 30 minutes. A film of ITO was 130 nm thick.

After the glass substrate having the transparent electrode line wascleaned, the glass substrate was mounted on a substrate holder of avacuum evaporation apparatus. Initially, the compound HI-1 wasevaporated on a surface of the glass substrate where the transparentelectrode line was provided in a manner to cover the transparentelectrode, thereby forming a 50-nm thick film of the compound HI-1. TheHI-1 film serves as a hole injecting layer.

After the film formation of the HI-1 film, a compound HT-1 wasevaporated on the HI-1 film to form a 60-nm thick HT-1 film. The HT-1film serves as a hole transporting layer.

The compound GH-4 (the host material) and the compound GD-1 (thefluorescent dopant material) were co-evaporated on the HT-1 film to forma 30-nm thick emitting layer. The concentration of the dopant materialwas set at 5 mass %.

An electron transporting compound ET-1 was evaporated on the emittinglayer to form a 25-nm thick electron transporting layer.

LiF was evaporated on the electron transporting layer to form a 1-nmthick LiF film.

A metal Al was evaporated on the LiF film to form an 80-nm thick metalcathode.

Thus, the organic EL device of Example 1 was prepared.

A device arrangement of the organic EL device in Example 1 isschematically shown as follows.

ITO(130)/HI-1(50)/HT-1(60)/GH-4:GD-1(30.5%)/ET-1(25)/LiF(1)/Al(80)

Numerals in parentheses represent a film thickness (unit: nm). Numeralsrepresented by percentage in the same parentheses represent a ratio(mass %) of an added component such as the fluorescent dopant materialin the emitting layer.

Evaluation of Organic EL Devices

The prepared organic EL devices were evaluated in terms of drivevoltage, CIE1931 chromaticity, current efficiency L/J, power efficiency,external quantum efficiency EQE, and delayed fluorescence ratio. Theevaluation items other than the delayed fluorescence ratio were measuredunder the current density of 1.00 mA/cm² and 10.00 mA/cm². Evaluationsof the results under the current density of 1.00 mA/cm² and 10.00 mA/cm²are respectively shown as Evaluation Example 1 and Evaluation Example 2.The results are shown in Table 2.

Drive Voltage

Voltage was applied between ITO and Al such that the current density was1.00 mA/cm² or 10.00 mA/cm², where the voltage (unit: V) was measured.

CIE1931 Chromaticity

Voltage was applied on each of the organic EL devices such that thecurrent density was 1.00 mA/cm² or 10.00 mA/cm², where CIE1931chromaticity coordinates (x, y) were measured using a spectroradiometerCS-1000 (manufactured by Konica Minolta Holdings, Inc.).

Current Efficiency L/J and Power Efficiency η

Voltage was applied on each of the organic EL devices such that thecurrent density was 1.00 mA/cm² or 10.00 mA/cm², where spectral radiancespectra were measured by the aforementioned spectroradiometer. Based onthe obtained spectral radiance spectra, the current efficiency (unit:cd/A) and the power efficiency ç (unit: lm/W) were calculated.

Main Peak Wavelength λ_(p)

A main peak wavelength λ_(p) was calculated based on the obtainedspectral-radiance spectra.

External Quantum Efficiency EQE

The external quantum efficiency EQE (unit: %) was calculated based onthe obtained spectral-radiance spectra, assuming that the spectra wasprovided under a Lambertian radiation.

TABLE 2 Current Luminous Density Voltage Intensity L/J η λ EQE (mA/cm²)(V) (nit) (cd/A) (lm/W) CIE-x CIE-y (nm) (%) Evaluation 10.00 3.971585.2 15.85 12.85 0.274 0.606 520 4.59 Example 1 Evaluation 1.00 3.44174.0 17.40 15.89 0.276 0.604 522 5.04 Example 2

As shown in Table 2, even when the current density was increased from1.00 mA/cm² to 10.00 mA/cm², the external quantum efficiency was notlargely reduced. Accordingly, it was recognized that the organic ELdevice of Example 1 emits light with a high efficiency even in the highcurrent density area.

Delayed Fluorescence Ratio

Voltage pulse waveform (pulse width: 500 micro second, frequency: 20 Hz,voltage: equivalent to 0.1 to 100 mA/cm²) output from a pulse generator8114A (manufactured by Agilent Technologies) was applied. EL emissionwas input in a photomultiplier R928 (manufactured by HAMAMATSU PHOTONICSK.K.). The pulse voltage waveform and the EL emission were synchronizedand loaded in an oscilloscope 2440 (manufactured by Tektronix) to obtaina transitional EL waveform. A value before the elapse of 10⁻⁵ seconds ofthe transitional EL waveform calculated by the method of least squareswas fitted in a linear line to determine a delayed fluorescence ratio.

Voltage of 0.14 mA/cm² was applied on the organic EL device of theExample 1 at the room temperature, where the transitional EL waveform isshown in FIG. 6A. The pulse voltage was removed at the time of about3×10⁻⁸ seconds.

In the graph of FIG. 6B, the voltage removal time is a starting pointand the inverse square root of luminous intensity before the elapse of1.5×10⁻⁵ seconds after voltage removal is plotted in an approximatelylinear line. The delayed fluorescence ratio of the organic EL device inExample 1 was 41% according to the graph. This delayed fluorescenceratio exceeds the theoretical upper-limit (37.5%) of the TTF ratio.

It was read from the graph in FIG. 6A that a residual intensity ratio in1 μs was 39.8%.

Relationship Between Current Efficiency and Current Density

The organic EL device of Example 1 was measured in terms of the currentefficiency in accordance with changes of the current density. FIG. 9shows measurement results as a graph showing a relationship of thecurrent efficiency.

As shown in FIG. 9, the current efficiency was higher in the currentdensity area of 1 mA/cm² to 10 mA/cm² than at the current density of0.01 mA/cm².

Example 2

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured byGeomatec Co., Ltd.) having an ITO transparent electrode (anode) wasultrasonic-cleaned in isopropyl alcohol for five minutes, and thenUV/ozone-cleaned for 30 minutes. A film of ITO was 70 nm thick.

After the glass substrate having the transparent electrode line wascleaned, the glass substrate was mounted on a substrate holder of avacuum evaporation apparatus. Initially, the compound HI-2 wasevaporated on a surface of the glass substrate where the transparentelectrode line was provided in a manner to cover the transparentelectrode, thereby forming a 5-nm thick film of the compound HI-2. TheHI-2 film serves as a hole injecting layer.

After the film formation of the HI-2 film, a compound HT-2 wasevaporated on the HI-2 film to form a 125-nm thick HT-2 film. After thefilm formation of the HT-2 film, the compound HT-3 was deposited on theHT-2 film to form a 25-nm thick HT-3 film. The HT-2 film and the HT-3film serve as a hole transporting layer.

A compound BH-1 (a host material) and a compound BD-1 (a fluorescentdopant material) were co-evaporated on the HT-3 film to form a 25-nmthick emitting layer. The concentration of the dopant material was setat 4 mass %.

An electron transporting compound ET-2 was evaporated on the emittinglayer to form a 5-nm thick hole blocking layer.

ET-3 and Liq were co-evaporated on the hole blocking layer to form a20-nm thick electron transporting layer. A concentration ratio betweenET-3 and Liq was set at 50 mass %:50 mass %

Liq was evaporated on the electron transporting layer to form a 1-nmthick Liq film.

A metal Al was evaporated on the Liq film to form an 80-nm thick metalcathode. Thus, the organic EL device of Example 2 was prepared.

A device arrangement of the organic EL device in Example 2 isschematically shown as follows.

ITO(70)/HI-2(5)/HT-2(125)/HT-3(25)/BH-1:BD-1(25.4%)/ET-2(5)/ET-3:Liq(20.50%)/Liq(1)/Al(80)

Numerals in parentheses represent a film thickness (unit: nm). Thenumerals represented by percentage in parentheses indicate a ratio (masspercentage) of BD-1 and Liq.

Evaluation of Organic EL Devices

The prepared organic EL devices were evaluated in terms of drivevoltage, CIE1931 chromaticity, current efficiency L/J, power efficiency,external quantum efficiency EQE, and delayed fluorescence ratio.Evaluation items other than the delayed fluorescence ratio wereevaluated as Evaluation Example 3 in the same manner as in Example at1.00 mA/cm² of the current density. The results are shown in Table 3.

TABLE 3 Current Luminous Density Voltage Intensity L/J η λ EQE (mA/cm²)(V) (nit) (cd/A) (lm/W) CIE-x CIE-y (nm) (%) Evaluation 1.00 3.81 57.625.76 4.75 0.13 0.197 471 4.16 Example 3

Delayed Fluorescence Ratio

A transitional EL waveform was obtained in the same manner as inExample 1. A value before the elapse of 10⁻⁵ seconds of the transitionalEL waveform calculated by the method of least squares was fitted in alinear line and was analyzed to determine a delayed fluorescence ratio.

Voltage of 1.00 mA/cm² was applied on the organic EL device of theExample 2 at the room temperature, where the transitional EL waveform isshown in FIG. 10. The pulse voltage was removed at the time of about3×10⁻⁸ seconds.

Based on the graph, where the voltage removal time was a starting pointand the inverse square root of luminous intensity before the elapse of1.0×10⁻⁵ seconds after voltage removal were plotted in the same manneras in Example 1, a delayed fluorescence ratio was obtained. The delayedfluorescence ratio of the organic EL device in Example 2 was 38.7%. Thisdelayed fluorescence ratio exceeds the theoretical upper-limit (37.5%)of the TTF ratio.

Residual Intensity Ratio in 1 μs

It was read from the graph in FIG. 10 that a residual intensity ratio in1 μs was 36.3%.

Reference Example

Herein, the organic EL device described in Document 2 are shown as areference example and compared with the organic EL device of Example 1in terms of the device arrangement.

A device arrangement of the organic EL devices in the reference exampleis schematically shown below in the same manner as in Example 1.ITO(110)/NPD(40)/m-CP(10)/m-CP:PIC-TRZ(20.6%)/BP4mPy(40)/LiF(0.8)/Al(70)

Compounds used in the reference example will be shown below.

The device only exhibits the maximum EQE of 5.1% in the current densityarea of 0.01 mA/cm² which is much lower than the current density area ina practical use. Accordingly, in a high current density area around 1 to10 mA/cm², roll-off is generated and a luminous efficiency is reduced.

Accordingly, it is recognized that the organic EL device of Example 1emitted light with a high efficiency even in the high current densityarea.

1-8. (canceled) 9: An organic electroluminescence device, comprising apair of electrodes and an organic compound layer between the pair ofelectrodes, the organic compound layer comprising an emitting layercomprising a first material and a second material, wherein: the secondmaterial is a fluorescent material; singlet energy EgS(H) of the firstmaterial and singlet energy EgS(D) of the second material satisfy arelationship of a formula (1):EgS(H)>EgS(D)  (1); and the first material satisfies a relationship of aformula (2) below in terms of a difference ΔST(H) between the singletenergy EgS(H) and an energy gap Eg_(77K)(H) at 77K:ΔST(H)=EgS(H)−Eg_(77K)(H)<0.3 [eV]  (2) 10: The organicelectroluminescence device according to claim 9, wherein the organicelectro luminescence device exhibits a ratio of the luminous intensitydue to delayed florescence relative to the total luminous intensity oflarger than 37.5%. 11: The organic electroluminescence device accordingto claim 9, wherein: the organic electroluminescence device exhibits aresidual intensity ratio larger than 36.0% after the elapse of 1 μsafter voltage removal in a transitional EL measurement; and the residualintensity ratio is a ratio of the luminous intensity due to delayedfluorescence relative to the total luminous intensity after the elapseof 1 μs. 12: The organic electroluminescence device according to claim9, wherein a half bandwidth of a photoluminescence spectrum of the firstmaterial is 50 nm or more. 13: The organic electroluminescence deviceaccording to claim 9, wherein a half bandwidth of a photoluminescencespectrum of the first material is 75 nm or more. 14: The organicelectroluminescence device according to claim 9, wherein a difference ΔTbetween the energy gap Eg_(77K)(H) at 77K of the first material and anenergy gap Eg_(77K)(D) at 77K of the second material satisfies arelationship of a formula (3):ΔT=Eg_(77K)(H)−Eg_(77K)(D)≧0.6 [eV]  (3).